text:
Why do all airliners have life jackets?
According to this question, life jackets are not mandatory in planes that do not fly over large bodies of water. However, I have never been on an airline flight without life jackets (and the corresponding safety demonstration).
Why do airlines not remove the life jackets from planes that fly over land? The plane would be lighter and the safety demonstration shorter. Moreover, it can not be hard to dedicate different planes to different routes and avoid returning the life jackets each time the plane has to fly over water.

The manpower and paperwork costs for removing the life jackets and then putting them back in outweigh potential cost savings.

You seem to miss the point that areas with no lakes or big rivers, but enough room to land (no mountains or very uneven area) are few and far in between. So as Flight 1549 showed the situation can happen anytime.

Good point @ThorstenS. You'd have to factor in the potential for diversions as well. If a flight has the potential to cross a large body of water they'd have to be on board.

I think the question is based on a false premise. Thousands of commercial flights operate every day with no life jacket or accompanying safety demo.  I'm mostly thinking of US domestic flights.  Also, many large craft have successfully ditched with survivors over the past 100 years or so, the benefit of life jackets on over water flight is very high compared to the cost, which is pretty miniscule compared to maintaining the engines, for example.

I can imagine it's also a psychological thing as a lot of people would probably feel better knowing they're there, even though they are useless.

@ThorstenS. 1549 would have had to carry lifejackets anyway, since approach and departure from NYC often crosses Long Island Sound and CLT-LGA flights are usually routed over Chesapeake Bay and Delaware Bay. (LGA-CLT, as 1549 was, tends to be routed a little farther north, over land.)

You don't have to look too hard at the history of airline incidents to see that even a nominally "overland" flight can end up in a body of water. The famous US Airways flight 1549 "miracle on the Hudson" from not long ago, and the tragic 1982 crash of Air Florida 90 into the Potomac River are just the ones I can remember off the top of my head. In the USA, many approach and departure patterns fly over water, and many flights pass over the Great Lakes. Also the same equipment usually goes on to fly other legs after a 100% overland leg.

@DavidRicherby Arrival and departure over water aren't generally an issue for commercial flights with/without life jackets, since flying over most bodies of water won't take you 50 miles from shore, as I understand the regulations. However, in the US, some routes over the Great Lakes, Gulf routes, and offshore routes on the east coast will get you more than 50 miles from land (not to mention, say, all the Caribbean destinations), and life vests would be required for all of those.

@Zach, it's not possible to be more than 50 miles from land in any of the Great Lakes. There's exactly one spot in Lake Superior that is just about exactly 50 miles from land.

The plane would be lighter.

Not by much.

It cannot be hard to dedicate different planes to different routes.

This adds to the logistics difficulties in scheduling the airframes. It's MUCH easier if all the airframes can be used on all the routes.

Returning the life jackets each time the plane has to fly over water.

This involves extra down-time/logistics before the airframe can fly the next trip.
You've used the phrase, "large bodies of water"; think about if the airplane needed to ditch in a "small body of water". It would be preferrable that all people had life jackets and thus a chance to live.

Maybe in addition to the logistical overhead of loading / unloading of life jackets there would also be the issue of significant wear and tear on them due to such frequent handling ?

Why do airlines not remove the life jackets from planes that fly over land?

They do, at least some of them. I fly on intra-European routes regularly (mainly Eurowings) and have never seen life jackets on flights that don't cross the Mediterranean. Obviously, the safety instructions don't include the life jacket demonstration either.
I would guess that especially for low-cost carriers, the weight advantage makes a difference.

How hard did you look for them? They often are not visible unless you search for them

Seats bases are flotation devices on some aircraft, so you might not realize you have one.

@GdD They're flotation devices, yes, but they're not life jackets.

Sure @DavidRicherby, but they satisfy requirements in some places.

@GdD I know. But the question is specifically about life jackets.

@DavidRicherby Was it?  Considering the OP claims to have never been on a flight without life jackets, they may be using it as a generic term for flotation devices.

@CarlKevinson Sorry, but doesn't it seem more likely that "life jackets" means "life jackets"? There are now two answers that say that some planes don't have life jackets but do have other flotation devices; that hasn't prompted the asker to say "Oh, I was including all flotation devices." The asker's main points are simplified safety demonstrations and reduced weight, and both of those are achieved by using seat cushions instead of life jackets. Surely the asker would have mentioned that if they knew about it? And if the asker's from the UK, all flights really do have literal life jackets.

Because this can still happen.

That’s US Airways Flight 1549 on the Hudson River. They made a movie about it starring Tom Hanks.
There’s no way to predict disasters, and had the plane been “efficient” having no life jackets, I suspect it could have meant a lot of people drowning. [But see comments.]
So it’s better to plan for contingencies, especially when it’s a cheap solution (Like life jackets or floating seats). In other words, planning for anything and everything going wrong is just something pilots and aerospace engineers (and lawyers) are in the habit of doing, ideally.

While I don’t agree that a lot of people would have drowned if there hadn’t been life jackets (many boats in the area responded very quickly to pick up passengers and there were other floatation devices available), this is still an excellent example of why it’s wiser to just have life jackets and floatation devices available on all flights.

@ToddWilcox thanks for the clarifying comment! (And also why I caveated my opinion with “suspect.”) ;-) I’ll make a minor edit though.

@Todd Wilcox: But if the boats hadn't been able to respond so quickly, the lifejackets might have been quite useful.

Although (as your photo shows), most passengers did not take a life jacket! Of the 150 passengers on flight 1549, only 33 took a jacket (although many more took a - useless - seat cushion) and only a fraction of those put them on correctly.

@GrayTaylor thanks for clarifying things Gray! I guess want your comment illustrates is that people should listen to the in flight instructions ;-p And fortunately, everyone was okay, and not over the ocean far from rescue.

Why do airlines not remove the life jackets from planes that fly over land? The plane would be lighter and the safety demonstration shorter.

Some do. As a specific example, if you fly Delta from Minneapolis to Lincoln, Nebraska, you'll be on a Canadair RJ-something (actually run by Sky West) that will only have a couple of infant lifejackets on board. Everyone else can use their seat bottom cushion to help float – there are straps on the bottom to help you hold onto it.
However, it only makes sense to do this on planes that won't be used over any significant body of water. For example, the lack of lifejackets prevents Sky West using those particular planes for something like a Minneapolis–Detroit flight, which would either pass over Lake Michigan or take an uncompetitive detour to the south. They probably couldn't be used on Minneapolis–Chicago flights, even, since those can be routed out over the lake to approach O'Hare from the east. Similarly, a European carrier that decided not to carry lifejackets would be unable to fly that plane to the UK without spending the time to put the lifejackets back and replace all the safety cards.

And, obviously, some regional carriers may still decide to keep the planes equipped anyway for route flexibility. I was on a SkyWest CRJ-200 from Flagstaff to Phoenix (119 miles over mostly desert) and they still gave the life jacket safety demonstration, although the flight attendant put a comical amount of emphasis on the “in the EXTREMELY UNLIKELY event of a water landing…” line.

The Airbus A380 seats around 850 passengers 4.  American Airlines has a fleet of about 950 aircraft of various models 2.  The Cadillac of Wal-mart life-vests, purchased individually, costs \$30 1.  Outfitting a fleet of 950 A380s with life-vests purchased individually at Wal-mart comes in a $24,225,000 USD. Even though I outfitted one of the largest fleets consisting of the largest aircraft with the life vests purchased using idiotically low business-sense, if I managed the rest of the business with some sense then the cost is (optimistically) about a day's worth of profits (American Airlines had a net profit of \$7.6B in 2015, though 2016 was a little slower at \$2.7B 3) for a once-in-an-aircraft's-liftime purchase (though there's no reason to retire them with the aircraft).
Regardless, your question was "why", not "why not"; what incentives are there for doing having life vests?  One possible incentive for airliners to go ahead and equip all aircraft with them is to take out any restrictions on the aircraft's serviceable zone that would be in-place if not equipped with life vests.  Maybe airports such as New York require life vests because it's so close to a large body of water that traveling over it is practically unavoidable?  Maybe it's economically advantageous to be able to re-route aircraft without the luxury of predictability.
Still I apologize I still have not fed you with any true answers, only supposition.  Maybe there is a regulation...Otherwise, maybe the CEO of American Airlines will respond and tell us why the hell he blankets the life vests.
(Edited for coherence).
Mins raised a point about cost of maintenance being a significant cost (possibly larger than the cost to supply).  So maybe the there isn't much strength in the "why not" court.
References:

I’d assume the life jackets used likely cost more than $30, given that they’ll be nicer than the nicest ones at Walmart (self-inflating, probably has a locator beacon, certified for flight which will add $$$). But your point still stands — the life jacket cost is going to be such a small fraction of the airline’s costs that in most cases it would be far more important to have the additional route flexibility of having them than the minuscule cost savings of not.


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How to enter an inverted spin?
Spins cannot occur without stalls. To enter a spin, a pilot would hold the elevator up and let speed bleeds off. This is how you'd approach a stall. Just as the stall starts to develop, apply full rudder; now the world is spinning around you.
Now, how does one enter an inverted spin? How do you stall an aircraft while it is inverted?

do a half-roll first and then do the same steps?

@ratchetfreak except stick forward?

When inverted, cut power. As speed bleeds off, gradually move stick forward keep your flight level, the nose will come up quite a bit over the horizon. It is important not to start sinking here as that will prevent the speed coming down low enough. When the stick hits the forward stop, kick in full rudder either way.
Enjoy!

What can also be done if the aircraft has sufficient pitch authority is to fly it into an upright stall, but to push full forward suddenly just before the wing stalls. When inverted, keep pushing and add rudder.

Yes, that is also a fun variation. kind of a spin inversion. Something similar can happen when you intend to break an upright spin but hang on to opposite rudder too long and push stick forward a little to much. If you do not understand what is happening here going from upright spin to inverted spin you might think you are still spinning upright  and the spin recovery did not work. This confusion is slightly less fun. The rule is, look forward over the cowling, not up through the canopy to see which way you are spinning.

Some aircraft exhibit unusual spin characteristics whereby the aircraft can transition multiple times from erect to inverted and vice versa, including reversal of direction, even with controls released. Not all spins are characterised by stable rates of rotation. Some are highly oscillatory and can also depend on entry conditions such as tail slide entries and power settings above idle thrust.

The easiest way to enter is the same as for an upright spin.
Cut engine. Bleed off speed. Keep altitude by pulling gradually. When buffeting indicates approaching stall, push stick decisively forward to mechanical stop and rudder pedals to one side. Same as you would enter a negative snap roll.
This is a clean and gentle entry, with a minimum radius, keeping the assigned box. Spectators do not recognise positive or negative (inverted) spin. But for the pilot, recognizing the direction of turns and recovery action is quite a bit easier.
Indications here apply for symmetrical, fully aerobatic airplanes.

Welcome to Aviation.SE! Nice answer.

So two things you need for a spin entry, as you mentioned, are a stalled wing and large enough yaw rate. If you stall the wing inverted (meaning pushing the control column forward, instead of pulling in a classic upright stall) it really doesn’t matter at what attitude you achieved this two requirements. But the method used in trainings is as @Wirewrap explained in his answer, because it is less oscillatory and a fully developed spin can be achieved in a couple of turns.
A word of caution though; inverted spins can be very disorienting and are prone to wrong recovery inputs by the pilot. So they should not be attempted without proper training by an instructor (like anything in aviation). Good news is it’s really difficult to find yourself in one, accidentally. There is almost no situation in normal flight requiring you to apply negative g and rudder at the same time (except maybe while recovering from upright spin). I think that’s why it's not demonstrated even at CFI trainings.

After I bought a Pitts S1 but before I flew it I got instruction in an S2A using the front seat for about the same sight picture. As well as circuits my instructor insisted on some upper airwork. He demonstrated how mishandling the aeroplane after long vertical climbs, with high power and low airspeed, that the little bugger could bite. A poorly executed stall turn, (US hammerhead) resulted in a rapid entry into an inverted spin as did a positive humpty with hands and feet off the controls. Propeller effects provided the yaw on that one. I did not see much warning on either of these.
In my own machine after a year or so I built up the courage to do some inverted spins myself. The right rudder ones worked well, but the first left rudder attempt did not spin and I ended up in an inverted spiral dive!
To get my S1 to do a left rudder inverted spin (the fast one) I have to apply right aileron after the rudder to get it started.
So I recommend some caution and plenty of altitude, and know what to do if you find yourself starting to wind up in a spiral with increasing airspeed while upside down. Hint: Don’t pull back!
Right rudder upright and left rudder inverted spins are steeper and faster than the other way with right-turning engines and can be made to go faster still by moving the stick off the stop. eg the inverted spin is started with full forward stick and full (left) rudder and accelerated by moving the stick rearwards after the spin gets going. Aileron is not needed after the spin has started properly.
Flat spins must be done with left rudder upright and right rudder inverted with right turning (Lycoming) engines.
Video of an accelerated right and left rudder inverted spins in link below. Not me.
[https://m.youtube.com/watch?v=FpKaFlR98zE]

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What is the difference between Assumed Temperature and Derate takeoff thrust?
Airlines like to avoid a full thrust takeoff as it increases the wear on the engine. There are two methods to achieve this: Assumed Temperature and Derate.
In the Assumed Temperature method, the takeoff performance is calculated by commanding the engine to mimic the thrust produced if the outside temperature is higher. Derated takeoff thrust basically tells the engine to pretend it is a weaker engine.
Both methods result in a takeoff thrust setting that is lower than 100% power. What are the differences between these methods, and how do the flight crew choose between the two?

I suspect the answer is it depends on which performance tables your FCOM/QRH/EFB has. But I am not a pilot and can't say for sure.

FWIW, in the 1980s and 1990s on 727s and 747s at the two carriers I flew for, we used the assumed temperature method. The flight engineer looked it up on paper tables, entered the info on a card, and put the card upright on the center console forward of the thrust levers, leaning it against the instrument pane.

On some aircrafts if you start by selecting an assumed temperature and then you change your mind and decide to select a % derated takeoff thrust, the final derated thrust percentage will be with respect to the assumed temperature thrust and not with respect to the 100% thrust. That will produce a cumulative effect with much lower thrust. I have experienced  this on the simulator; finally I asked a type pilot who confirmed that it is like that on this aircraft, and that for him and for his colleagues it has been recommended to use only assumed temperature.

The difference is in the minimum control speeds ($V_{mcg}$ and $V_{mca}$).
In Assumed Temperature takeoff thrust, the pilots are allowed to advance the thrust levers to full rated takeoff thrust. However, in Derated takeoff thrust, the levers should not be increased beyond the fixed derate limit, otherwise an engine failure may result in loss of directional control.
From Boeing 737 FCTM (my emphasis):

The fixed derate is considered a limitation for takeoff. Takeoff speeds consider ground and in-air minimum control speeds (VMCG and VMCA) at the fixed derate level of thrust. Thrust levers should not be advanced beyond the fixed derate limit unless conditions are encountered during the takeoff where additional thrust is needed on both engines, such as windshear. A thrust increase, following an engine failure could result in loss of directional control.

The minimum control speeds are calculated based on the full rated takeoff thrust. The rated thrust is lowered in Derated takeoff but not in Assumed Temperature takeoff. Therefore, by calculating the minimum speeds based on a lower engine rating, the pilots can be ensured that sufficient directional control can be maintained on a slippery runway, whereas there is no such guarantee for Assumed Temperature takeoff.
On a dry and long runway, there is no operational difference between Assumed Temperature and Derated takeoff.
From the Boeing 777 FCTM (my emphasis):

Reduced takeoff thrust (ATM) may be used for takeoff on a wet runway if
approved takeoff performance data for a wet runway is used. However,
reduced takeoff thrust (ATM) is not permitted for takeoff on a runway
contaminated with standing water, slush, snow, or ice.

And:

Derated takeoff thrust (fixed derate) may be used for takeoff on a wet
runway and on a runway contaminated with standing water, slush, snow, or
ice.

The Boeing 737 FCTM has a nearly identical description.
The A330/A340 FCTM explains that the slower $V_{mcg}$ / $V_1$ can also allow increased MTOW on short runways:

When taking off from short or contaminated runways where ASDA [Accelerate-Stop Distance Available] is the limiting factor, a reduction in the minimum control speeds may generate a take-off performance benefit and a higher MTOW.

I didn't mean to answer my own question. I was waiting for some airline pilots to shine some light on the information I have, but since (apparently) no one has...

The statement that you can't do an assumed temperature takeoff with standing water, slush, snow, or ice MAY be specific to the 777 rather than universal.  Clearly, with stuff on the runway, lots of power is a good thing, but I think there are some operations that can do assumed temperature with at least some of those, if all other parameters are met.

@RalphJ I believe the concern is the reduced friction on the runway, which lowers the tire's traction to counter an asymmetrical thrust condition until sufficient airspeed is achieved.

@RalphJ FWIW, in the 1980s and 1990s on the 727 and the 747 at the two carriers I flew for, we were not allowed to do reduced thrust takeoffs on contaminated runways. As I remember, when the contamination was just rain, it was the captain's call as to whether it constituted standing water.

General
The actual takeoff weight of the aircraft is often lower than the maximum regulatory takeoff weight. In this case, it may be possible to takeoff at a thrust less than the maximum takeoff thrust. This allows to increase the engine life, improve the engine reliability and reduce the maintenance costs.
Two categories of takeoff at reduced thrust exist:

The use of flexible temperature concept referred to as "flexible takeoff"
The use of a fixed derated thrust level referred to as "derated takeoff"

Flexible Takeoff Definition
When the actual takeoff weight is lower than the maximum performance limited takeoff weight, the aircraft may comply with the regulatory requirements with a reduced thrust, called "flexible takeoff thrust".
This takeoff operation is the "FLEX takeoff".
Flexible Takeoff Principle
The FLEX takeoff principle is based on the change in maximum available thrust with OAT.
The maximum performance limited takeoff weight depends on the maximum available takeoff thrust, therefore it is possible to determine a temperature at which the actual takeoff weight would be limited by performance.
This temperature is referred to as "TFLEX (Flex Temperature)".

Flexible Takeoff Limitations
Takeoff at reduced thrust, so-called as "FLEX takeoff", is allowed only if the airplane meets all performance requirements at the takeoff weight, with the operating engines at the thrust available for the flexible temperature (TFLEX).
TFLEX cannot be:

Higher than TMAXFLEX
Lower than the flat rating temperature (TREF)
Lower than the actual OAT

FLEX takeoff is not permitted on contaminated runways.
Some items listed in the MEL and CDL do not permit a flexible takeoff.
The operator should check the maximum thrust (TOGA) by:

Performing full-rated takeoffs at regular intervals, in order to detect a reduced EGT margin, or
Maintaining an adequate engine monitoring program, in order to follow-up on the engine parameters

Derated Takeoff Priniciple
The derated takeoff enables to improve the takeoff performance if the TOW is limited by VMCG.
VMCG limitation usually occurs on short and/or contaminated runways. The principle is to impose a lower engine rating to benefit from lower minimum control speeds.
In the case of derated takeoff, the minimum control speeds (VMCG and VMCA) are decreased because:

The derated thrust is lower than the maximum takeoff thrust
The effect of temperature on maximum available thrust is taken into account

VMCG Limiting
VMCG only has an impact on the ASD that includes an acceleration and a deceleration phase.
The decrease in VMCG allows to use lower V1 values in the speed optimization, which shortens both the acceleration and the deceleration distances.
The weight decrement linked to the derated thrust is largely compensated by the benefit of shorter ASD. As a consequence, if takeoff performance is limited by the VMCG limited weight, an overall improvement can be achieved by derating.
VMCA Limiting
The penalizing effect on climb gradient of the decrease in thrust outweighs the advantage of a lower VMCA.
Therefore derated takeoff would not improve the takeoff weight if VMCA limited.
It is not permitted to combine a derated takeoff and a FLEX takeoff.
When a derated takeoff is performed, the selection of full takeoff thrust, by setting thrust levers to TOGA, is not permitted below the speeds specified in the "Engine Failure After V1 - Continued Takeoff Operating Technique". The only exception is the Recovery Technique at takeoff after the penetration of unforecasted windshear.

This looks like it was copied from somewhere. If so, you should mark it as a quotation (use ">" symbol in markdown) and add a source.

Dar, the answer is Airbus-centric and possibly pertaining to a particular operator's aircraft. Regulatory certification is based on the options chosen by the operator, and the resulting actual configuration. For example, on current Boeing jets and even going back to the B747-200/300 it is very much allowed to use the Assumed Temp (AT) on top of T/O Derates. Is it possible for you to differentiate text taken/extracted/quoted from the Airbus manual and the text written by you? This would help give a conclusive answer.

Please also look at this link for Use of Assumed Temp alongwith T/O Derates on Boeing jets

Possible source for the Flexible Takeoff info (taken from the A380 manual?): https://m.blog.naver.com/PostView.nhn?blogId=fltops&logNo=221040917975 Possible source for the Derated Takeoff info (ref A340 performance): http://carlosneto.org/fs/files/A340PERFORMANCE.PDF

Both methods will give you an N1 setting that's less than what you'd have if you used actual temperature & no derate (i.e. max takeoff thrust).  The computations to get to that N1 setting are different, but at the end of the day, the pilots (or the autothrottle) sets a particular N1, and you go fly.  Which method, derate or assumed temperature, or both, that you used to get to that N1, isn't really all that important.
For the pilots, the company will probably tell them which method to use.  Or the performance computations will consider whatever permutations are out there, and spit out an answer -- derate to XXk, assume temp of YY degrees, or maybe both.  And the pilots put that into the FMC, and you go fly, knowing that the N1 that you get at the end gives you enough performance to do all the things you need to do: 35' at the end of the runway if you lose one at V1, meet obstacle clearance criteria, etc.
The derate method is typically less granular than assumed temperature -- you have two or maybe three derate setting you can select, and that's that.  But those steps may be pretty big steps.  The software can step an assumed temperature up one degree at a time to find exactly the minimum power that's needed to meet all the criteria.  But there is some limit to what temperature you can assume, and if you have the really light airplane on a really long runway, even assuming that max temperature may give you lots of performance to spare, and so in order to bring the thrust down to being only what you really need and no more, you need the bigger "bite" of a derate.
There is a human factor to consider, in that if the crew is selecting a derate and entering an assumed temperature, you want to be sure that they did that process right, and (particularly if conditions or runway change) you don't end up with the wrong combination of derate + assumed temperature.  If Plan A was to derate two steps down & use the actual temperature, then Plan B was to derate one step down & assume 45 degrees C, the hybrid of not changing the derate (leaving it at two steps down) AND assuming 45 degrees C will give you less thrust than either Plan A or Plan B, and you want to make sure that an error like that can be reliably trapped.
But again, that sort of decision of what method(s) are used is made more by the company and the performance software than by the individual crew.  As a pilot, put what you're given into the box, verify that it's right, and go fly.  I'm not overly concerned which computational path the software used along the way to telling me to set 88.3% N1.

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text:
How many executive jets does the US military have access to (as opposed to owning outright)?
Last night around 8pm at least 12-15 executive jets flew into PSM over my house one after the other. Apparently the Air Force brass were having some kind of powow, which isn't surprising given that the new refueling tanker has just been deployed to Pease so they are probably having a lot of reviews and evaluations.
However, after jets kept flying in I started thinking how many of those things do they have? According to the Wikipedia, the Air Force only has 18 (rather antiquated) C21As, which I find hard to believe, since if that is the case, the whole fleet flew into Pease last night. Also, I find it pretty hard to believe that they don't have newer jets.
I suspect that the reason why the true number of jets is not listed, is that the list only includes executive jets that the Air Force owns, and does not include those that it leases. Are there any easily available statistics on aircraft leases by the Air Force?

Perhaps they weren't USAF. Could have been USAPAT. Flightaware would likely have given you an indication.

They do have 18 C21As (ave age 33.3 yrs).  They also have 9 C-37As aka Gulfstream V (ave age 17.7 yrs), and 3 C-37Bs (ave age 8.7 yrs).  I'm not aware of any leased a/c operated by the AF.  It is common for them to contract flight service where the plane is operated by the contract crew. These will have N-numbered a/c.  Try checking Flightaware.  AF inventory numbers are from Air Force Association's AF Magazine Almanac issue.


text:
Are there any problems opening a helipad close to an airport?
I am looking at the possibility of installing a helipad at my house which is within 1.5 miles of a Class D airport. The location is not in the approach to any of the runways, however.
I am not a rotorcraft guy, so I am wondering if having a helipad this close to an airport will cause any issues?
(note that the whole area is Class B airspace, so it will always require a clearance to take off)

Is this is for Sweden or Canada?

@mins yeah, yeah, very funny.

Unless you are within the inner ring of the Class-B you don't need a clearance to take off VFR if you stay below the Class-B shelf... Out of curiosity, why, not being a rotorcraft guy, would you want to have a helipad on your property? Probably won't do you any favors with the neighbors.

In the US, normally it is not a problem.  I would talk with the tower supervisor, to see if a planner could be consulted before you go too far down the road and start paving or something like that.
A good place to start would be a memorandum of understanding, where you and the FAA would work out the operations out of the field, and discuss arrival and departure coordination.
I could come up with a list of twenty heliports within 1.5 miles of Class C or D airports. Funny how hospitals end up near airports. Ditto for police facilities.
Life is easier if you do not plan on trying to get instrument approaches.


text:
What is the purpose of using 8.33 kHz instead of 25 kHz frequency spacing?
What is the purpose of using 8.33 kHz instead of 25 kHz frequency spacing?

More context would be very helpful.  Is this for air-to-ground communication, or something else?

I edited the question & title to match what I think you're after. If I'm wrong, please feel free to revert the change.

A good reminder that digital is useless!

If you are referring to 25 kHz spacing versus 8.33 kHz spacing, it gives more channel options in radio transceivers.
Ex1. There are 41 channels in the frequency space of 118.0 and 119.0 MHz, inclusively, with 25 kHz spacing.
Ex2. There are 121 channels in the frequency space of 118.0 and 119.0 MHz, inclusively, with 8.33 kHz spacing.
Since current airband transceivers are analog instead of digital, we needed some way for accommodating the increased amount of use on the airbands. We use analog receivers to this day because of their nature of transmitting and receiving as much as possible regardless of the situation or environment.  The transmission may be faint, staticky or garbled, but it can still be received.
On the other hand digital transceivers like cell phones can use much fewer frequencies to accommodate many times the amount of information and users by compressing the information. A digital transmission is usually much clearer and more distinct with less distortion even over great distances with lower power. But, digital transmissions are an all or nothing proposition. Any loss or interruption in the flow of the data may render that portion of the information unreceivable. One word in a transmission may be entirely lost, changing the entire meaning of the transmission without the receiver knowing.
Ex. “Descending one five thousand” is a different meaning with different consequences than “Descending five thousand”.
25 spacing is used in the United States. Europe and other countries use 8.33 spacing. Many NEW avionics and airband  transceivers feature user selectable spacing.  Some transceivers are available in 25 spacing and 8.33 spacing models, with the 8.33 models often being more expensive to purchase.

A vote for explaining the reason for stickin' with the ancient analogue tech and chopping the channels once more

"Digital transmissions are an all or nothing proposition. Any loss or interruption in the flow of the data may render that portion of the information unreceivable" - that's just not true. Modern audio transmission protocols have enough redundancy built-in that loss of a few bits/packets will just result in a decrease of audio quality, not a complete dropout. You seem to hint at a usability issue though: "without the receiver knowing" would because of hearing silence instead of static in the case of an actual dropout, but that should be fixable.

@Bergi - Fixable but not fixed. Again, cell phones are a good example. At least with cellphones, you are expecting a certain flow in a conversation. Don’t get me wrong, you have the same problem with analog when a transmission gets stepped on over heterodyned over. There is a big movement to switch all av-radios to digital. It won’t happen quickly. There are too many aircraft and facilities in which to make the installation for too much money. The interim during the changeover would require a lot of coordination between governments. Some in third world countries without funds.

Nothing prevents analog signals from dropping out either, particularly if one of the stations is moving. The mitigation strategy here is read backs.

The purpose is to accomondate more dedicated frequencies within the airband VHF range (117.975 to 137 MHz).
Increasing number of stations made this necessary, if the smaller division was not implemented, it was estimated that only 70% of future requirements for frequencies in Europe can be met.
833radio.com has further information and an nice table describing the channel spacing:

For simplicity, the actual 8.33 channels are displayed as rounded to .005 values.
For example to use the frequency 118.0083MHz, you dial 118.010

Those are some awfully confusing nominal channel frequencies—the 8 kHz channel whose actual center frequency is 118.000 has a nominal frequency of 118.005! It looks like the nominal frequencies are the actual center frequencies rounded to the nearest 5 kHz, except that if the center frequency is the same as the center frequency of a 25 kHz channel, then 5 kHz is added to the nominal frequency in order to distinguish the 8 kHz channel from the 25 kHz channel.

@TerranSwett Then again, for the operator of the radio it makes very little difference what the actual frequency for any given dial is. The reasons for the 8.33 division are technical, while the reasons for dial are psychological.

From Wikipedia:

Channel spacing for voice communication on the airband was originally 200 kHz until 1947, providing 70 channels from 118 to 132 MHz. Some radios of that time provided receive-only coverage below 118 MHz for a total of 90 channels. From 1947–1958 the spacing became 100 kHz; from 1954 split once again to 50 kHz and the upper limit extended to 135.95 MHz (360 channels), and then to 25 kHz in 1972 to provide 720 usable channels. On 1 January 1990 the frequencies between 136.000 and 136.975 MHz were added, resulting in 760 channels.
Increasing air traffic congestion has led to further subdivision into narrow-band 8.33 kHz channels in the ICAO European region; all aircraft flying are required to have communication equipment for this channel spacing. Outside of Europe, 8.33 kHz channels are permitted in many countries but not widely used as of 2012.

As already stated in other answers, the narrower the spacing, the more channels which can be fit within a given spectrum. Channels have to have some amount of spacing because signal modulated onto a carrier will create sideband; the exact nature of the sideband depends on the signal to be modulated and the type of modulation. In general, some sideband is inevitable because it actually represents the input signal. Rejecting sideband in a receiver generally excludes information (lower quality of demodulated signal). In order to allow for multiple channels to co-exist, transmitters must confine their sideband emissions so as not to interfere with transmissions on adjacent channels and receivers have to be capable of recovering the transmitted signal with acceptable fidelity; this means that receivers have to accept some amount sideband while rejecting frequencies too far away from that assigned to the channel.
The progression from wide spacings to narrower spacings reflects technological improvements in radio design; transmitters improve to confine their emissions to narrower sidebands while receivers improve to recover acceptable signals from the narrower transmissions. Congestion of the RF spectrum is what drives to need to increase the number of channels, either by allocating new portions of the spectrum or by fitting more channels into the existing spectrum.


text:
What is the meaning of this airport marking?

This mark is identified as a plane control marking and based on an apron surface. What does it mean?

The marking indicates that vehicles must yield to aircraft. It is placed in areas where a service road crosses a taxiway.

Just seems like common sense to yield to the 20 ton difficult to steer behemoth but at least with the sign there's legal liability. Like us getting a failure to yield ticket.

@corsiKa At that point, the ticket seems like a case of "arson, murder, and jaywalking".

@corsiKa In aviation we have a thing for redundancy and additional layers of safety :)

Re, "common sense..." Seafarers often call that "The Law of Gross Tonnage."

@corsiKa, it is common sense to yield to the 200 ton behemoth, but the point of the sign is to remind you that you are about to enter area where those behemoths regularly move and therefore this is the right place to stop and check you are not going to run into any.

Useless knowledge: the airborne counterpart of a behemoth is a Ziz. Don't mess with them either.

It's a stop sign. Implying stop, then yield.
Here, a left-right access road crosses an up-down taxiway.
The sign directs street vehicles, not airplanes.
In US/DOT vernacular, widely copied:  shape is everything.  All octagonal signs are stop signs, just as all inverted triangle signs are yields, all round signs are RR approaches, and all crossbucks are RR tracks.  They can be snow covered, they can have a clarifying suffix such as "Eating Meat", they can be paisley, they can be green and say "Go". They all mean "stop, then yield". The shape overrides the art.
Obviously, this is painted on a surface, rather than on a standing sign, for operational reasons. But the choice of octagon, coupled with red with white border design language, does everything possible to invoke the standard DOT style of a highway traffic stop sign. It would be grossly negligent for any surface vehicle operator to do anything other than stop and search for something to yield to.
Note that a standard STOP sign gives no clue as to what exactly you are yielding to.  The driver is obliged to figure that out from context.   (Although sometimes on the highway, DOTs will add nearby clues outside of the stop sign proper; here there would be a supplemental sign indicating "taxiway" in words or graphics).
This particular STOP sign is non-standard, and no DOT would install it on a highway.  However the design language makes the purpose crystal clear: Stop, then yield for something... With a big hint as to the typical "something".
And the design is a bit whimsical; obviously the yellow line does not separate two distinct lanes of airplanes as the graphic might suggest.


text:
What is the purpose of the propeller?
The propeller generates thrust, but what is the purpose of thrust of propeller?  Is it to help the aircraft move forward or to produce lift to rise up?

You may read how it flies?, especially the sections about energy conversion. It is a good further reading to understand how an aircraft move and how pilot manage both altitude and speed variations.

I'm not trying to be rude at all, but what about the function of the propeller confuses you?

@quietflyer no ,i don't intend for purpose of thrust,obiviously it a force opposite to the resistive drag force,i am confused by the purpose of propeller?

@MichaelHall  whether it/propeller is used to raise/lift airplane or to move it to forward direction or both functions?

I guess I am confused by the level of current understanding.  Your user name is "engr" which implies that you might be an engineer.  However, the question is not phrased like an engineer would ask.  So I don't know if you are looking for an engineering explanation, or a simple "it makes the plane go forward" like you would explain it to a child.  The function of a prop can be understood readily by building a rubber band powered balsa model.  I'm just struggling to understand what you already know...

Question reads like a homework question to me.

The propeller's job is to keep the pilot cool;. If the propeller ever stops, the pilot will sweat profusely!!

The propeller generates thrust to counter drag.
Drag is created by both the forward velocity of the plane through air (parasite drag) and as a byproduct of lift (induced drag).
Lift is a also a consequence of movement through air and is used to counter weight.
The above is mostly true for conventional, heavier than air, fixed wing aircraft in un-accelerated flight. More complex cases like VTOL aircraft use thrust to directly counter weigh during some stages of flight.
From Wikipedia:

A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is proportional to the mass of the airstream multiplied by the difference in velocity of the airstream. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades, or using a thrust reverser on a jet engine. Rotary wing aircraft and thrust vectoring V/STOL aircraft use engine thrust to support the weight of the aircraft, and vector sum of this thrust fore and aft to control forward speed.

You mean that propeller help/supports to move aircraft in forward direction??and not to produce lift( for rising upward)??

@engr yes, that is what usually happens, the propeller moves the airplane forward and keeps it at a speed at which the wings can produce enough lift. Only helicopters and other, more niche, aircraft use thrust to directly counter weight.

Isn't the airspeed over the wing higher directly behind the propeller?

Here's a simple example. Without a propeller, a glider moves forwards and (in calm air) descends very gently.  So lift is a result of moving forward.
Then, moving forward is a result of pointing the nose slightly down -- or, if the glider has a sustainer engine, a propeller.
So the primary purpose of a propeller is to keep the airplane moving forward.

Could say something like -- "Then, moving forward is a result of pointing the nose slightly down -- or more precisely, of following a descending flight path through the airmass."  Probably too much detail already for such a simple question, but helps avoid the apparent paradox of the plane or glider descending with nose pointing up as discussed here https://aviation.stackexchange.com/questions/70842/how-does-an-aircraft-descend-without-its-nose-pointing-down/70848#70848 .

Could say so, but the OP wants clarification of the basics, which justifies the simplification.

Fair enough, it is a very basic question.

Basic, ya think?!  Trying to contain myself here, but the basic function of a prop is something most people understand pretty early on...

Lift is not necessarily the result of moving forwards, but of air moving over the wings.  So it's possible (at least if you're a bird) to soar without moving relative to the ground.  And with a helicopter, you can have the "wings" moving around in a circle while the rest of the machine stands still.

@jamesqf -- "Lift is not necessarily the result of moving forwards, but of air moving over the wings. So it's possible (at least if you're a bird) to soar without moving relative to the ground."  -- in the context of fixed-wing flight, that is just a reference-frame issue, obviously.

@quiet flyer: Obviously to some of us, but in answering such a basic question, I think it needs to be made explicit.

The propeller generates thrust, but what is the purpose of thrust of
  propeller? Is it to help the aircraft move forward or to produce lift
  to rise up?

The most concise answer would be "Yes, both."  Even though thrust and lift are different things, you can't sustain a climb without thrust, and if you try, you'll run out of airspeed, which means you'll also run out of lift.  And you can't sustain a climb without lift, either.
Your question though very simple opens the door to thinking about what is meant by words like "purpose" and "cause" in the context of physical forces.  All the forces at play in a given physical situation (e.g. a linear climb at constant airspeed) can in some sense said to be helping to "cause" that situation.
Yet in another sense, though lift is always present in a climb (other than a purely vertical one), it is not really the the "cause" of the climb.  Read on for more--
A propeller's thrust exerts a forward force on the aircraft, acting mainly in the opposite direction as the drag vector.  Yet even if no thrust is present, an aircraft can maintain airspeed just fine (at least up to its terminal velocity) by using gravity to offset drag.  This means that the flight path must be descending, at least in relation to the airmass.  An aircraft flying without any thrust can't maintain altitude or climb, at least not without trading off some airspeed, assuming no updraft is present in the airmass.  So thrust is always needed for sustained horizontal or climbing flight in non-rising air.
One misconception that may be embedded in your question is a hint of an idea that we may need more lift to climb than to fly level or descend. Actually, lift is important regardless of whether we are gliding downward, flying horizontally, or climbing upward.  A climb does not require more lift than level flight.  It's just more complicated to generate lift when we are climbing or flying level than when we are descending, because it's harder to sustain the required forward motion, because we aren't getting any help from gravity.  Hence the need for thrust.  So your intuition that a propeller has something to do with generating lift to allow climbing flight has some truth to it, but not because we need any more lift to climb than to fly horizontally or to descend.
Some of these things were said in a simpler way in another good answer to your question: What is the purpose of the propeller? .
If you want to ask "what happens if the propeller falls off", (ignoring the resulting CG shift), then the answer depends on what constraint we apply.
Is the flight path constrained to be horizontal or climbing?  Then the airspeed will decrease until the stall speed (stall angle-of-attack) is reached.
Or is the airspeed constrained to be constant?  Then the aircraft will start to descend.
Or is the wing's angle-of-attack constrained to be constant?  Then the aircraft will start to lose airspeed, the flight path will curve downward, the aircraft will go through several cycles of a pitch "phugoid" oscillation, and the aircraft will finally stabilize in a descending glide at a slightly lower airspeed than it had in powered level flight.
The pilot's control inputs will determine which of these cases apply, or some other case that is a hybrid of two or more of them.
The ultimate answer to your question, which appears to be essentially "what is the effect of thrust", ultimately lies in an understanding of the vector diagrams showing the forces acting on an aircraft in various situations.  Such a diagram will give a better answer than any number of words.
So you may want to learn how to draw the force vector diagrams that apply for unpowered gliding flight, powered horizontal flight, and powered climbing flight.  These related answers may help:
What produces Thrust along the line of flight in a glider?
'Gravitational' power vs. engine power
Does lift equal weight in a climb?
John S Denker's "See How It Flies" website ( https://www.av8n.com/how/ ) is great resource for anyone wanting to gain a detailed understand of the physics of flight.  However it is aimed at people that already have some understanding of flying on the practical level, so it may not be the best resource for a complete beginner. This is the section that addresses the balance of forces in level flight, gliding flight, and climbing flight: https://www.av8n.com/how/htm/4forces.html#sec-four-forces .  Note that Denker has not made the simplifying assumption used in the other links and comments above, that thrust acts parallel to the flight path through the airmass.
(It appears possible that you may simply be asking a question about which direction the propeller's force vector points in various modes of flight.  If so, that could be better addressed with a different and more specific question.  While thrust is not always defined as acting purely opposite to drag, as a rule of thumb the direction of the force from the propeller does act mainly parallel (and opposite) to the drag vector, with only a small component acting parallel to the lift vector.  Bear in mind that sometimes engines are mounted with a small amount of down-thrust, so in high-speed cruising flight a propeller's force might actually include a very small component acting opposite to the direction of the wing's lift vector.)

Other links could be added to this answer that would be more appropriate for an elementary, beginner-level approach to the question--

Nothing wrong with your answer, but it may be a bit much for a basic question like this. You know... What's a CG shift? What's a stall speed/angle-of-attack? etc. Maybe those will lead to new questions from the OP who seems to be starting from near zero on the phenomenon of flight. (Not that there's anything wrong with that, either.)

I know, can't help myself.

future edit: change "lift is important" to "lift is essential"


text:
How about inventing a "fuel" that will not erupt into fire?
One of the most concerning factor in flight safety is the possibility of the plane erupting into fire due to a fuel spill and a spark. Is it possible---or have researchers ever worked on---to power an aircraft with something that does not set the plane afire?

Lots of things will burn in the right conditions, including avgas, jet fuel, and lithium batteries. I suspect that this is fundamentally a physics/chemistry issue: if any substance contains a lot of energy in a limited weight/volume - which is what you need in aviation - then by definition it will 'want' to release that energy if the conditions are right. I think your question is interesting and relevant but if you don't get a good answer here you might try physics.SE and/or chemistry.SE.

What makes fuel useful as, well, fuel, is that it DOES burn, thus releasing lots of energy.  It seems unlikely that one could find something you can use to power an aircraft, as fuel, which releases lots of energy when burned in the engine, but which won't do the same when burned outside of the engine.  The high flashpoint of Kerosene means that fuel spills aren't all that much of a big deal, but with sufficiently high temperatures it certainly will burn. Nuclear propulsion has been tried, but it doesn't work out well -- energy density is too low.

@CannonFodder oh man I remember seeing that on TV!

You might try going with a bipropellant where the fuel and oxidizer are non-reactive with atmospheric oxygen. You'll at least halve the fuel efficiency of the engine, but hey, it won't burst unless you mix the two!

"One of the most concerning factor in flight safety is the possibility of the plane erupting into fire due to a fuel spill and a spark" According to whom?

Yes, of course, inventors and researchers have always been looking into alternative ways to power planes.

Rising air: Gliders successfully stay in the air using thermals, ridge lift, lee waves or convergence zones.
Spring power. Springs can be made of various materials including rubber and steel. Rubber spring power is still popular in toy planes
Solar power. Experimental planes have been flown successfully with  solar cells and electric motors.
Human power: The very first planes were human powered, and there are still inventors trying to push the limits
Nuclear power:

Fission: The energy density of nuclear fuel like Uranium-235 is even higher than that of chemical fuel. Experiments with fission reactors have been done up to working engine prototypes. The development has been abandoned. The radiation during operation and after waste disposable, leakage, and the risk of uncontained failure are unacceptable. Nuclear fuel does not burn, but fission reactors can melt. Sub systems containing steam can explode.
Fusion: While stationary experiments for fusion reactors are ongoing, so far no practical reactor has been implemented. Fusion does generate radiation as well, but to much less extend and risk than fission. The hot plasma in a fusion reactor cannot grow out of control. However, we still have to see a successful stationary implementation before thinking seriously about fusion powered planes.

Apparently none of first four power sources is used for commercial air transportation, because they are unreliable and/or limiting the range. Fission nuclear power could provide reliability and range, but radiation issues forbid the use of this technology. Fusion remains science fiction for time being.
Instead, chemical fuels are used because of their very high power density, their perceived abundance, and low cost.
Any power storage of high density is potentially dangerous. Just imagine a massive spring that would have enough mechanical energy stored to power a heavy long range aircraft. In case of a mechanical failure, the full amount of energy would be released in an instant. That would be just as disastrous as chemical fuel going into flames.

Treat this as you like, but one of the direct answers to this question (have researchers ever worked on to power an aircraft with something that does not set the plane afire?) is - go nuclear.
It is still possible to start a fire from overheating, but not ball of fire, and not from a sparkle. And don't worry about nuclear explosion: it is remarkably hard to make one, even having a piece of fully enriched uranium or plutonium at hand.
There will be other problems, of course, but nothing is free :)

Exactly. If there is anything to add, it would be that nuclear planes did exist, and had a bunch of other interesting benefits, such as being able to stay airborne during months without “refueling.”

Biodiesel has a flashpoint as high as 130-140 Celsius. This makes it far less likely to burst into flames.
Ultimately, anything useful as fuel will burn. Even batteries burn - and violently so. It's not possible to eliminate the possibility, but it's possible to greatly reduce it.
Another way to reduce this propensity is structural. There are two major parts to it: reducing the likelihood and the amount of spillage in a moderate crash; and insulating the fuselage against exterior fires.
Currently the focus is on making a post-survivable-crash fuselage survive an external fire long enough for the passengers to evacuate. Needless to say, when you're dealing with a crashed airplane, things will never go according to plan - but at least it upgrades the plan from "everyone dies" to "people die where things other than the crash itself also go wrong".

Is it possible---or have researchers ever worked on---to power an aircraft with something that does not set the plane afire?

Yes. It’s called JP-5. It’s a version of JP-4, that has a higher flashpoint (60 deg C vs -18 deg C). JP-5 was developed for use by the USN, specifically to have a higher flash point for increased safety on aircraft carriers.
JP-4 has now been replaced by JP-8 (flashpoint => 38 deg. C). JP-5 is still in use.
See: this DTIC presentation.

The DTIC presentation is great!  I learned a ton from that.  Nice find.


text:
Who designed and built Rans S-7 airplane?
I read in the Washington Times, that a man named Swingle, who has no knowledge and experience in aircraft design, built one which he called a bush airplane. But I read in Wikipedia, that the RANS S-7 series seems different as it was built by a company, Rans Inc, seems after the founder name, Randy Schlitter.
Does anyone know the complete story about the aircraft, the engine specification, the size, the weight, the dimension, etc, of that Rans S-7 as mentioned in the Washington Post article? It is interesting as it can land in a savanna.

The Ran's S-7 is a kit plane. The article didn't clearly state it, but the gentleman from Ohio bought a Rans kit and assembled it himself, he didn't design the aircraft, he put a kit together.

Oh I see. I though that he build himself, as mentioned in the article he need 21 months to finish his airplane.

He did build it, but he didn't design it. You can buy kit planes and assemble them yourself.

What is that mean built but didn't design? Is like he used someone's design?

He bought a kit, complete with blueprints (plans). Not unlike buying a model car that you glue together. All the pieces and instructions were provided to him, he just followed them and put the parts together.

So, it is clearly that he didn't build it? If he just follow a design, can not be said that he built it his own.

He did build it though. He just didn't engineer it. Just because you use parts/plans doesn't mean you don't build it.

Where did the design he get? Is it from Rans Inc?

Yes, that is part of the kit that he buys. He gets all the needed parts and the plans, he takes those plans and puts it together to assemble an airplane. Rans sells the parts and the plans, he builds it.

@AirCraft Lover: Perhaps the distinction between "design" and "build" would be a question better suited for the English language site.

@AirCraftLover many companies sell DIY packages containing everything you need to build an aircraft except the tools (rivet guns, screwdrivers, etc.). Just follow the instructions and put it together, like a plastic model kit (but obviously a lot more parts and accuracy needed).

@jwenting, thank you for the info. What else company that sell such as kits?

How much it probably will cost if I buy it a complete kits include the engine?

@AirCraftLover Vans are a large supplier. Total cost depends heavily on your selected options (specific aircraft type, engine, avionics, etc.).

@AirCraft Lover: Use Google (or other search engine, if you prefer).  About $25K for the basic kit.

@jamesqf, yes my friend, I will. I just curious actually, and curios if any from all of you have bought one.

Here is the webpage for purchasing a Rans S-7S Courier airplane.
https://www.rans.com/s-7s-courier
You can buy a kit that needs assembly, or you can by an already assembled airplane (Ready to Fly).
Final cost depends on the options you select for the kit, or for the factory assembled airplane.

Thank you my friend. Very interesting.


text:
Is there a standard for the frequency/interval between flashes of anti collision strobe light?
As far as I've spotted on lots of aircraft, I've noticed that the anti collision flashes are in a similar interval, no matter if it's a Boeing 737, Airbus or an F-16 Fighting Falcon. Is there any international standard or any sort of aviation safety rules that standardizes the frequency of the anti collision strobe light? If there any such rules, does it regulate whether the lights should have a single or double flashes at a time?

For the FAA, this can be found in 14 CFR Part 25 - AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES, Subpart F - Equipment, Lights (§§ 25.1381 - 25.1403):

§ 25.1401 Anticollision light system.
  (a) General. The airplane must have an anticollision light system that -
(1) Consists of one or more approved anticollision lights located so that their light will not impair the crew's vision or detract from the conspicuity of the position lights; and
(2) Meets the requirements of paragraphs (b) through (f) of this section.
...
(c) Flashing characteristics. The arrangement of the system, that is, the number of light sources, beam width, speed of rotation, and other characteristics, must give an effective flash frequency of not less than 40, nor more than 100 cycles per minute. The effective flash frequency is the frequency at which the airplane's complete anticollision light system is observed from a distance, and applies to each sector of light including any overlaps that exist when the system consists of more than one light source. In overlaps, flash frequencies may exceed 100, but not 180 cycles per minute.

Similar sections can be found for other types of aircraft in other sections; the specified flash rate is identical for all of them.
US military aircraft are not bound by FAA rules, of course, but they tend to voluntarily comply with them anyway unless they have a good reason not to.


text:
How are compressor disks connected to the shaft?
I am trying to learn how compressor disks and shaft are connected each other. I don't have any idea about the terminology so I couldn't find anything useful.
Here is a section view of a sample and I indicated the connection area. What type of connection elements are used there? Anything related will be helpful.

Well the thing with the red arrow pointing at it in your picture is not part of the compressor at all.  The red arrow is pointing to a bolt that holds a bearing housing onto the fan frame.  it's a non-rotating component.  Are you sure that is the area you are interested in?  If you are interested in connections between compressor disks you need to move just slightly aft (right on the picture).

Your question:

Here is a section view of a sample and I indicated the connection
area.

What type of connection elements are used there? Anything
related will be helpful.

From the title of your question I guess you assume the bolted element is a HPC rotor, and you might be wondering why a rotor would be bolted to a stationary element, the fan frame.
Indeed this would make no sense, and the explanation is the area you point to is not a HPC rotor but the rear seal support of the forward sump, a stationary element which role is to contain oil within the sump. The linkage is indeed a series of bolts.
The explanation below is related to this sump seal, but as you will see it also incidentally explains how disks are connected to the shaft as this plays a role in the sealing.

Rotors in the engine are centered within two frames, the fan frame and the turbine frame. The fan frame in addition has the role of transferring engine thrust to the wing. These two functions (centering and load transfer) are implemented using bearings requiring cooling and lubrication. Oil is used, and contained around bearings by closed sumps.
Forward sump
The area in your question is the end of the forward sump, located within the fan frame. The sump contains bearings #1 and #2 at the front and #3 at the rear. Bearing #3 is actually made of a ball bearing for thrust transfer to the fan frame and the mount point on the pylon, and a roller bearing for centering the HPC shaft. Here is a view of this portion for a CFM56-7B:

Forward sump - CFM56-7B, source: CTC 215, page 34
If you are not familiar with the bearings, sumps or seals, or the engine mounting points, the following questions will help:

In a turbofan what holds the spinning axis?
How does a labyrinth seal work when the engine isn't rotating?
What is a seal in aeronautics?
How are engines mounted onto wings?

Rear stationary seal
If we focus on the rear portion of the sump, we see the sump is closed by a seal between the fan frame and the HP shaft N2:

CFM56-7B, forward sump, rear stationary seal, source ibid, page 136
The seal is made of two elements: A stationary support centering seal tracks, bolted to the fan frame hub, and the labyrinth seal rotating with the HPC shaft. The bolts are the ones indicated by the arrow in your question. This looks like this:

CFM56-7B rear portion of the forward sump
The seal support closes the sump. A part of the sump enclosure is given by the fan frame: The fan hub. Note also the red (magenta) N2 shaft which is bolted to the 3rd stage rotor and centered in the fan hub by the seal (the load is actually on the bearings within the sump).

By the way this answers you second question about how are the HPC disks centered on the shaft. HPC rotors disks are grouped into 3 elements, stage 1-2 single spool, stage 3 disk, stage 4-9 single spool. The two spools (they look like drums) are bolted to stage 3 disk which is the only element centered on the shaft as explained above (but if you want more details, please read In a turbofan what holds the spinning axis?). Contrary to other engines, in the CFM56-7B there are no separate disks, except for stage 3, other disks (with labyrinth seal teeth and dovetail slots for blades) are cast and machined as single spools. From CTC 215 already mentioned:

CFM56-7B HPC rotors, spools and shaft

A picture of the stationary part, with the tracks in white (that's the best picture I could find):

Stationary air/oil seal, source
The labyrinth rotating ring is fixed to the IGB horizontal bevel gear sleeve which is used to spin the accessory box.

IGB location, source
In the picture above we can see:

The HPC shaft (red) with its conical front end bolted to the stage 3 rotor of the HPC.
The IGB gears (blue), the horizontal one being fixed to the shaft
The sump cavity (green).

The stationary seal just close the green cavity by linking the shaft to the fan frame (the frame has been removed to show the sump).
See this answer for more.
Oil is prevented to leave the sump by the labyrinth teeth, but this is not perfect, so air is also injected into the labyrinth to push oil back into the sump. There are holes in the labyrinth ring to let air in and oil out. Air and a bit of oil fog not eliminated by the centrifugal filter are drained into the center pipe running up to the exhaust where they mix with hot air from the turbine (path used is indicated on the drawing in your question by arrows).
From Airbus, the role of the seal is:

The rotating air/oil seal provides the sealing of the aft end of the
forward sump and acts as a locknut for the No 3 bearing.
Holes between the two aft seal teeth allow passage of sump
pressurizing air. Holes passing underneath the forward seal teeth
allow oil to be drained into the No 3 bearing cavity.
The rotating air/oil seal has internal threads for installation onto
the horizontal bevel gear and its front

This is a good post, but I think they were interested in the compressor disks, not the sump, even though they pointed at the sump bolt.

@OSUZorba: From the title it seems indeed the question is related to the HPC, but from the drawing it seems related to the forward sump seal and the OP wants to clarify the reason a rotor would be bolted to the fan frame as this would make no sense. My answer now include both aspects (and  I already detailed how rotors are centered in another question I also linked).

In effect, this question is related to In a turbofan what holds the spinning axis?. This question has an excellent answer that closely relates to this question (but isn't a duplicate). The following figure is re-posted for a relevant answer. Credits go to user mins.

HP rotors of CFM56-7B. Adapted from CFM56-7B Familiarization Manual

This answer hints to the given figure of the F110-GE-129, but may be equally valid for other engine types (as seen in the image above for a CFM56 turbofan engine, the image is very similar, this is because the CFM56 core comes from the GE F101, which was the basis for the F110), note that some differences are found, but the general idea to assemble the rotor disks are similar.
The encircled area contains the first high pressure compressor rotor disk (it looks as if it is dangling freely, but remember that the disks are rotationally symmetric, so this is basically a ring), this disk is bolted or more commonly, inertia welded (reducing structural complexity and the number of parts), to the next (second high pressure) compressor disk, which is connected the the next, etc. Sometimes, the disks are sets, so multiple disks are connected to another set or welded all together. Note that the high pressure turbine rotor is (or sometimes rotors are) also connected to this drum.

The inertia welded 12-stage F101 (and the CFM 56) compressor
rotors have only five structural parts and two bolted flange joints.

from: Bernard L. Koff, Gas Turbine Technology Evolution: A Designer’s Perspective, JOURNAL OF PROPULSION AND POWER, Vol. 20, No. 4, July–August 2004
The third (high pressure compressor) disk expands or is connected to a hub (in the image above designated to be the N2 shaft). This hub contains a spline interface that is connected to the high pressure shaft, or makes up the high pressure shaft (such an interface is also present at the turbine disk, so there are two points where the drum attaches to the bearings). Basically, all rotor disks form a drum with an extension to mount the drum to the shaft to form the spool.

+1, clear explanation of the rotor system.

I have found detailed cross-sectional diagrams can be somewhat confusing. Here is a video from a Canadian gas turbine mechanic who has hundreds of videos of their components & assembly methods. I think the answer to your question is a combination of splines, & nuts & bolts. If this video doesn't satisfy you, then search the rest of his channel, it is quite extensive.


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Why does the Cessna 152/172 nose wheel turn only when the aircraft is moving?
According to the POH, ground control is achieved by a spring loaded steering bungee.
But how does it work and why the nose wheel does not move when the aircraft is stationary?

Because the nose wheel is only attached to the rudder pedals indirectly, through some springs (or maybe a bungee cord), that transfer the displacement of the pedals into a force that is applied to the position of the wheel. In a typical cross-wind landing scenario, where the aircraft would touch down with the nose wheel off center, that eliminates the possibility that the aircraft would turn hard to one side when the nose is lowered because the nose wheel is cocked to one side (due to the rudder pedals being displaced due to the crosswind). Because the force is only applied through the tension on the springs, the aircraft inertia (tendency to continue moving in the same direction), overcomes the springs and aligns the nose wheel with the ground track when the rubber touches the ground even though the rudder pedals are displaced.
The nose wheel does not turn when the aircraft is stationary, because there is no inertial force at work, and the friction of the wheel on the ground keeps the wheel from turning (the spring force is not strong enough to overcome the friction)

So this is the aircraft reason why it's set up this way. Thanks!

Re "That eliminates the possibility that the aircraft will land with the wheel off-center (like in a crosswind landing situation," - when the gear is unloaded, doesn't it drop down into a detent that locks it straight ahead?  So that it doesn't move if you push on the rudder pedals in flight?

That could be, I remember reading that retractable nose gears did that so they would fit into the wheel well, but I have not heard that before for fixed nose gear,  but even if it did do that, without the springs arrangement, if the wheel was directly connected to the rudder pedals when weight was applied, it would still cause an abrupt, dangerous turn when you let the nose down with pedals displaced.

@JohnK -- I'm sure I remember reading that at least some of the light Cessnas have a detent that centers the wheel when unloaded but I can't recall which aircraft, will try to to relocate that info--

My Long EZ has a retractable nose gear, but no nose gear steering, (it relies on differential braking). But the wheel still needs to be aligned with the fuselage when it goes into the wheel well. Obviously the wheel will be aligned with the fuselage just prior to rotation, and hopefully, streamlining airloads will keep it aligned as it retracts, but I always look through my little plexiglass window into the nose gear well as it retracts to make sure. (There is an adjustable anti- shimmy thing, which, if adjusted too tight, can prevent it from aligning properly)

Most of the high wing Cessnas have a cam that centers the nose wheel and locks it in that postiion when the strut is fully extended (bearing no weight).  This locking lets the bungees act as a centering force for the rudder while airborne.

See the 2nd picture in https://aviation.stackexchange.com/a/52510/38119 for an example on the 172.

@AShelly ah you're right I stand corrected.

The inertia is the inertia of the entire aircraft. Because it is moving, inertia keeps it moving in tbe same direction with the same velocity, (unless an external force changes its motion). The friction of the wheel, rolling on the ground surface then keeps the wheel pointed in the same direction.

@Chris, lets be clear. turning refers to what happens when you turn the steering wheel. rotating is when it spins on it's bearings. So yes, the inertia of the aircraft does keep the wheel from turning, (or causes it to align wiyh your ground track when it is not already aligned), because it is rotating on it's axis which is cambered, (tilted) forwards just like the front wheel on your bike or on your car.

@chris, nothing I said has anything to do with inflight. We are talking about what happens on the ground.  The question is "According to the POH, "ground control" is achieved by a spring loaded steering bungee. ..." In flight, the nose wheel will deflect according to the forces applied by the bungee cord (or springs) as controlled by the position of the rudder pedals, (unless there is a mechanical centering mechanism as mentioned by other answers).

On the ground, the wheel is rolling, and because of the inertia, it rolls in a straight line aligned with the ground track,, so yes, indeed, it does know what the ground track is.

@Chris, until you quoted that sentence from my answer, I did not understand your confusion. Re-reading that part of my answer,  I see what you were thinking and I agree. I will reword to clarify.

@CharlesBretana That's clearer, thanks. Though note that a 152 has a centering cam that keeps the wheel centered when weight is taken off the nose wheel, so the nose wheel will only be off center on touchdown if the nose is not aligned with the runway.

I'm not a pilot, nor have I ever been in a C172, but I'm going to go out on a limb here and answer from a logical physics point of view and say the answer is that it's because the nose-wheel steering is operated by a bungee cord.
When you step on the pedal, it pulls the bungee. Bungee cords, being by definition stretchy, will stretch. When the plane is stationary, the cord stretches, but there's more resistance between the wheel and the ground than the cord can overcome, so the wheel doesn't move.
When the plane is rolling, the bungee still stretches, but it wants to shrink back up. Since the wheel is constantly moving, it has less resistance against the pavement so as the bungee tries to shrink, it can actually pull the nose gear in the direction you want to go, thus you end up steering.
When you turn the steering wheel in your car, the wheels move whether you're rolling or not because there are steel links between the steering rack and the toe link on the wheel hub. Your movement of the steering wheel is directly fed (with very little rubber in between) to the wheel of the car. If you were to ever get into a car without power steering, you'd notice that it's much harder to turn the wheel when the car is stationary than when it's rolling because the stationary tire against the ground has much more resistance than when it's rolling.

If you were to ever get into a car without power steering ... or a car with the engine not running ... or a car with the fan belt torn ...

The simple answer is that a slowly rolling wheel has less resistance to sideways motion.
The spring is there to prevent imperfections in the rolling surface from jarring or stressing the control linkages to the rudder (especially with unpaved airstrips).  It provides some side force so the front wheel, as it rolls on its path, will change direction.

with infinite ruts, the odds off the wheel being pushed left or right become equal, allowing the spring to turn the wheel in the desired direction

Rubber, being an elastic polymer, will "sink in" to the little cracks and imperfections in the surface as several hundred pounds of aircraft weight sits upon it, increasing resistance to movement when the wheel is not turning.
Of course, at higher rotation speeds, gyroscopic effects will work against the pull of the spring, but the wheel will still turn.
Every once in a while, especially with insufficient yoke back pressure, the wheel can start resonating, causing a very unpleasant shudder while taxiing or on a take-off run.  Increasing backpressure or changing speeds can help, and sometimes it's best to abort and make sure it's nothing worse.

the more significant factor is the inertia of the moving aircraft, and the need to prevent significant nose wheel misalignment, (with the ground track), from causing the aircraft to suddenly turn hard to one side (ground loop?),  when landing with rudder pedals displaced.

Landing on the mains helps too.

Hah! Yes, excellent point! Although we're supposed to keep cross controls applied until slow enough that aerodynamic rudder is ineffective....


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Why are there so many moving parts used in the landing gear?
This short clip of the landing gear on a Hawker 850XP being retracted seems extremely complex.

I'm curious to know why there are so many moving parts but as a layman I'm struggling to make sense of it all.
So what exactly is going on here and why?

A good guess is some kind of complicated way of folding it into the compartment.

Compared to other landing gear I've seen, this is relatively uncomplicated.  You might want to edit it into a generic question rather than about the Hawker since your opening premise is wrong.

@simon, I'm open to suggestions as to how this can be changed. What would you say it is relatively uncomplicated compared to?

One of the key points (circled in red) above shows the part that locks the gear in the extended position and requires that the linkage up top (circled in blue) is actuated to release the gear.
At the end of the clip you can see a glimpse of the linkage bar at the top returning to its previous position. That way if the gear is dropped by gravity it will lock itself once fully deployed.
If you strip those parts all that's left is a basic 4-bar linkage and a connection to the gear door.


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Does any company (or country) still use Zeppelins or airships? Why not?
I just like them a lot when I see'em in movies and I was wondering, are they still being used anywhere? Are there any companies working on them?
If the answer is, as I expect, "Nope", I have to ask why?
(Update: the "why" is more or less explained here Why are airships not more popular? , I still hope the answer is not "Nope"!)
Naïvely I would've thought they should be useful for heavy cargo, passengers, cheaper and safer than planes...

Related: http://aviation.stackexchange.com/questions/9860/why-are-airships-not-more-popular

I don't know how I overlooked that. I'll add it to the question. The "why" is more or less explained there. I do wonder whether there are companies working on it though.

I think  the biggest problem with lighter than air craft is that they're ungainly in unstable wind conditions.

Zeppelins had a rigid skeleton and several internal gas bags. None of them are still in operation.
Airships today are blimps, that is, one big gas bag which is kept in shape by internal pressure, which in turn is regulated by ballonets.
The only airships flying today are used for

Advertising (Goodyear blimps, MetLife blimps)
Military observation (Lockheed Martin P-791)
Sightseeing tours (Zeppelin NT)

All attempts (and there were many) to use airships for cargo transport have eventually failed, sometimes spectacularly.

Don't forget the MetLife blimps (which like the Goodyear blimps serve a dual role: Advertising for the company operating them, and as a camera platform for sports coverage). The MetLife blimps (Snoopy 1, Snoopy 2, and Snoopy J) do a lot of TV coverage for golf events (see here).

Also worth a look: Lockheed-Martin's "Hybrid Airship" - I don't know that any are in actual use as cargo platforms, but that's their intent. We'll see whether they can make a successful go of it.

That was a bizarre contraption!

@voretaq7: The hybrid airship is actually the P-791. Cargo versions are planned with 3 to 7 times its actual size, but have not been built yet.

@PeterKämpf I'm sure they'll build at least one - think of the size of the Lockheed-Martin logo you can paint on that sad-looking manatee of an aircraft! :) I think the civilian cargo-hauling version is going to be the LMH-1, they signed a contract for a bunch of them in March but I don't think any will be flying until at least 2017/2018.


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What procedures are used for midair refueling without air supremacy?
One of the major themes of the answers to (and comments on) my earlier question about tankers not having ejection seats is that tankers are used only in environments where the threat from enemy aircraft is minimal or nonexistent; for instance:

...Tankers are big, slow, and devoid of defensive measures because they don't need any of that. They're used from a very safe distance, and if there is a risk of enemy air power, friendly air power runs interference...
...Primarily because tankers and transports are being operated in non-combat environments or safely benhind [sic] the front lines of a battlefield so ejection seats are not absolutely necessary for crew survival...
...In the end, it's unlikely a refueler will be shot down. Refuelers are generally only flown in areas where we have control of the airspace, or reason to believe that we are well away from enemy fire...
...A refueler will never be out on it's own. Like I said, they expect aerial supremacy, and have fighters run interference for them. If a fighter is closing on a refueler, full of tens of thousands of pounds of jet fuel, they're already gone...
...For the United States, the assumption is that we will always be able to have air superiority in some area due to overwhelming firepower and technology (though area that may be further away from the battlefield, making the tanker less effective, though still more efficient than landing.) The tanker would never fly directly over the combat zone unless we had air superiority there, as it's quite simply a sitting duck - they have basic defenses though, as you can never guarantee air superiority...
...Tankers are not supposed to be flying in any area where they would be taking enemy fire, so an evacuation situation would essentially be a freak event. Most of the scenarios where a tanker would be attacked have a low survivability anyway...

However, it is quite conceivable that combat aircraft might require mid-air refueling in a situation where air supremacy – or, worse, even air superiority - cannot necessarily be attained (for instance, in a full-scale war between the United States et al and China, or, less likely, Russia), and where restricting tanker aircraft to uncontested airspace would put them so far away from the target(s) being attacked as to be useless.
What are the procedures that are followed for refueling combat aircraft in an environment of mere air superiority or (worse) air parity, rather than air supremacy?  Are the types of tankers used restricted to only those converted from bombers (and thus already possessing both ejection seats and greater maneuverability compared to tankers based off of civilian designs)?  Do aircraft rely solely on buddy tanking (thus tying up most of one’s combat force with tanker duty)?  Are tankers escorted by dense enough swarms of fighters to be able to bull their way through any airspace, no matter how hostile (thus using up most of the tanker’s fuel merely to sustain its own escorts)?  Or do the ministries of war of the countries involved simply resign themselves to having to mass-mail “we regret to inform you” letters?

Where's the problem?
Air superiority changes locally. Somewhere, there will be a front line. Hopefully, the more you move away from it on your side, the higher air superiority you get, so at some distance it becomes viable to have your tankers.
An this will define your combat aircraft's action radius, which hopefully will still reach sufficiently beyond the frontline.
If you don't have air superiority anywhere, then you have bigger problems anyway and don't need to worry about tankers.

The idea that an area of "air superiority" is incredibly far from the battlefield is a misconception. You can have air superiority near enough to refuel, but far enough that you have enough time to counter the attack before it reaches your tanker in the "safe space".

The obvious short answer already given is fighter support.  Any answers specific to a theater of operations, and detailed enough to provide an answer you might deem sufficient, would be classified.

air-to-air missiles on the tanker?

Why won't refuelers operate in combat zones?
As noted in another answer, the United States has a range problem with their aircraft. Performance, stealth, weaponry, all come at the cost of fuel efficiency. This could also be a potential result of the fact that we have had aerial superiority in all of our warzones for many years for aerial refueling, and the Navy and Marine Corps often both have carriers nearby to return to. We often haven't needed better range, but in a state-on-state conflict, these avenues would become very difficult. Air superiority would be more difficult to attain, and aircraft carriers would become high-priority enemy targets.
In your question, you mention that perhaps refuelers could be sent into enemy airspace if they had ejection seats and greater maneuverability. This would still never happen. Unfortunately, but understandably, the military does not hold back on air missions due to human lives alone. All service members take a risk, particularly pilots, and that is an understood part of the job. Ejection seats hold a high risk of killing you either way, and if critical to mission success, the risk of human lives is one that the military has to be willing to take. The larger problem is that refuelers are large targets. If you have a refueler in a combat zone, the enemy will target it, not only because it is large, slow, and easy to shoot down, but also because it has a ripple effect. Shooting down the sole tanker that iw keeping multiple squadrons in the air could potentially cause the other aircraft to run out of fuel and force them either crash, or leave the commat zone. As the Air Force guys are taught: When you destroy the supporting assets, the fighting assets will naturally disappear.
What separates refuelers from bombers?
Bombers are not meant to be sent straight at an enemy squadron either. They often have to be. Because of this necessity, the bombers are built for stealth and/or speed. Take for example, the B1-B Lancer. This bomber flies above the speed of sound, making it harder to catch, track, or hit with missiles. There is also the B-2 Spirit, which uses stealth to arrive at it's target. At night, this could fly right overhead without anyone ever seeing it. Outfitting a refueler to go supersonic would be fairly silly, as it has to go at an awfully low speed during refueling anyway. A stealthy refueler is more possible, but stealth cover would still be blown during refueling operations - two aircraft connected by a hose is certainly more likely to pop up on radar. The cost outweighs the benefit, for now.
In short, a bomber needs to fly at the enemies, while a refueler does not.
What will a refueler do when air superiority cannot be attained?
A dogfight will not occur over the heart of enemy territory. Air operations have a range - refuelers will fly in a protected area, and the dependent aircraft will have to stay within a particular range of that area. If they can't reach it, they don't attack it. Refuelers require the assumption that somewhere, the nation will have aerial superiority, if only because there are no nearby enemies at that time.

It's actually a problem the US Navy at least has been considering - with improvements in anti-ship missile capability (such as China's Dong-Feng 21D) meaning that carriers are at risk at ranges approaching 900NMi having to keep the ships out of range is taking a large chunk out of the effective range of aircraft such as the F35C which has a range of 1,200NMi and refuelling a "stealth" aircraft with a conventional tanker plane sort of defeats the point.
As a House Armed Services Committee report noted:

The committee notes that the aircraft carrier air wing has been optimized for striking power and sortie generation and believes that it may not be configured to support the long-range strike required by current and future threat systems. While the introduction of the F-35C will significantly expand stealth capabilities, the F-35C could require increased range to address necessary targets. The committee believes that several options could be used to address this issue to include developing a stealth tanker capability, improved engine technology, or to develop and procure a strike capability that is purposely built to strike at increased range.

The current proposed answer to this is the MQ-25 development of which was approved by the US Navy earlier this year for service in 2024.
The MQ-25 is intended as an un-manned tanker with stealth capabilities delivering fuel for 4-6 aircraft at a range of ~500NMi, as a) a stealth aircraft itself it will be harder to detect and intercept and b) as UCAV if it does get shot down there's no need to think about ejection seats or any other mechanism to protect the crew.


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How to calculate and check if a/c can carry additional reserve fuel without affecting payload and if so how much?
Find payload and state if a/c can carry additional reserve fuel without affecting payload and if so how much?

Max take off weight= 45000 lbs.
Max landing weight= 40000 lbs.
Flight fuel= 4000 lbs.
Reserve fuel= 800 lbs.
Empty weight= 25000 lbs.

So, By calculating the take off weight using the above figures:

TOW = Landing weight + Flight Fuel = 44000lbs

Payload = TOW - (Empty Weight + Total Fuel)

Payload = 44000lbs - (25000 + 4000 + 800)

Payload = 14200lbs.

Now how should I calculate and check if a/c can carry additional reserve fuel without affecting payload and if so how much?

With no payload, your landing weight is:

LW1 = 25,000 (EW) + 800 (reserve) = 25,800 lbs

The maximum allowed payload, as regards to LW, is:

Payload = 40,000 (MLW) - 25,800 (LW1) = 14,200 lbs

With this payload and the trip fuel, the TOW is:

TOW = 25,000 (EW) + 800 (reserve) + 14,200 (payload) + 4,000 (trip) = 44,000 lbs

This TOW is 1,000 lbs lower than MTOW, so you can takeoff.

With this payload the new LW is:

LW2 = 25,000 (EW) + 800 (reserve) + 14,200 (payload) = 40,000 lbs

The margin for landing is:

LW margin = 40,000 (MLW) - 40,000 (LW2) = 0

There is no weight margin for landing. Therefore if reserve fuel must be added, the payload must be reduced, and vice-versa.

However if some more fuel is required per regulation, e.g. for an alternate destination or holding, then MLW will be exceeded or you'll have to burn it before landing. Perhaps this aspect is to be ignored for your exercise, else it's a trap.

If the flight fuel was computed for a takeoff weight of 44,000 and you add an extra 1,000 payload, you may need to account for an increased fuel burn.

@Terry: That's a very good point I missed.


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How do I have to fold a emergency slide so it can be reused?
I have the opportunity to buy a never used emergency slide from an Airbus A320 for cheap. So I need to know how they can be reused and how to deflate and repack them so I can use it for another time.

Are you planning to install this in another Airbus A320?

Your English is just fine (better than I've heard from many native English speaking Americans), don't apologize!

This would make an awesome Slip 'n Slide™ for summer-time backyard fun!!

Im not planing to install this in another a320 , I want to use it in my backyard just for fun and for my curiosity.

I don't think they are meant to be repacked anywhere other than the factory, so I'd be surprised if that information was available.

@RonBeyer maybe there are some training materials somewhere.

Watch one of these slides being deployed (and the video doesn't do justice to how loud and dramatic it is in person) and observe how quickly and forcefully it springs open. Watch what happens when you try to deploy one in a parking lot for fun (that's a larger slide, but the concept still applies). This is clearly not an activity that can safely be performed in your backyard, and serious injury or death could result.

WARNING
These slides may contain explosive squibs and/or chemicals not safe for ingestion and it is NOT SAFE to mess with them if you do not know what you are doing! If an airbag in a car can kill or cause injury, imagine an enormous version of that. If one became available for sale, I would be very cautious as to whether it had been made safe/inert. Tread carefully.
Only authorised life support fitter type maintainers at the factory or the approved delegated authority will be able to fold slides up again to the correct standard. It is a specialist type job and a jigsaw puzzle. It is similar to trying to get a tent back in it's bag if you don't use the correct technique.
As with all aircraft maintenance tasks, it may sound like a cop out, but the answer to any how do you do that question is "in accordance with the approved procedure". Find the approved procedure and you will know. It will be in the airbus manual or from the manufacturer of the slide.

Altough your warnings are useful, the only piece of answer you bring is basically "go get a approved maintainer" without giving a clue of how it is done (which is the question). You should add material about that as it is the asked question.

To you two naysayers, sometimes the best answer is a warning.  Now if he said it had been used, and he was just looking for suggestions on how to re-inflate it for backyard use, that would be different.  But we do have some responsibility to not condone careless actions.

Yeah, I don't really care if I don't answer it to your standards. I've made the poster aware that it is dangerous. The standard you walk past is the standard you set. I'm obliged to make people aware of unsafe actions. I would think this is a very understandable mindset given it is a website about aviation.

@craig Agreed. If the question was 'How do I re arm this ejection seat' nobody would look twice at your answer. Sometimes it's a case of "If you have to ask, don't do it"

The slide is just a slide and can be packed and inflated without pyrotechnics.
We use slides for training; we use them on the ground to demonstrate them as rafts, and to side down.
Manufacturer documents are used to maintain the slide, and will include packing instructions.
The slides are not hazardous to handle, neither to inflate nor repack. The exception would be if the slide is armed with the rapid inflator; that's something that should be handled appropriately.  Otherwise, they can slowly be inflated.
For home use, I'd seek modification from a firm that does inflatable carnival or yard toys like a bounce house, that can bond a proper filler valve and make the slide usable for the longer term.
Be aware that aircraft slides aren't nearly as slick as one might think; going down a slide, you're quite likely to get friction burns on exposed skin, and I've burned a hole in uniform pants on them, as well as hurt my leg and hand.
If you're just using it for home entertainment, then inflating it and using it for bouncing, or things like that, are probably fine.  I suppose it could be put on a lake, but they're not really meant for repeat use after that.